Handling

Let's consider Python tools that help us to control code execution and errors.

Error Handling Structure

There are different keywords are used to control program errors:

try: this keyword is used for the code block where we expect the error.
except: this keyword is used for the code block executed if an error is raised.
else: this keyword is used to perform certain logic if errors are not raised.
finally: this keyword is used for the code block that always executes after all structure.

try and except

These are the main keywords for error handling, the try code block waits for errors, and the except catches it:


              12345678
            
print("Code above is executed")

try:
    1 / 0
except:
    print("Error is catched")

print("Code below is executed")

The interpreter is not stopped after the ZeroDivisionError raising because the except keyword caught it.

Note
Don't try to put all your code inside the try block - this will lead to unexpected consequences and can break the program.

If the try block is executed without errors, the except block will not be executed.


              1234
            
try:
    print("No errors")
except:
    print("Error is catched")

The try block does not roll back executed code. All the code lines before an error are executed:


              1234567891011
            
lst = []

try:
    lst.append("first")  # executed
    lst.append("second")  # executed
    1 / 0  # error
    lst.append("third")  # not executed
except:
    print("Error caught")

print(lst)

Note
The try block raises the exactly one error because executing stops after an error.

else and finally

The else and finally blocks are optional and used less often.


              12345678910
            
try:
    print("try: Start")
#    1 / 0
    print("try: End")
except:
    print("except: Error is raised")
else:
    print("else: The try block is executed succesfuly")
finally:
    print("finally: Always executed")

You can add and remove comments in the example above to understand how it works.

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